Sub-cycle temporal evolution of light-induced electron dynamics in hexagonal 2D materials

Abstract

Two-dimensional materials with hexagonal symmetry such as graphene and transition metal dichalcogenides are unique materials to study light-field-controlled electron dynamics inside of a solid. Around the K-point, the dispersion relation represents an ideal system to study intricately coupled intraband motion and interband (Landau-Zener) transitions driven by the optical field of phase-controlled few-cycle laser pulses. Based on the coupled nature of the intraband and interband processes, we have recently observed in graphene repeated coherent Landau-Zener transitions between valence and conduction band separated by around half an optical period of 1.3 fs (Higuchi et al Nature 550, 224 (2017)). Due to the low temporal symmetry of the applied laser pulse, a residual current density and a net electron polarization are formed. Here we show extended numerical data on the temporal evolution of the conduction band population of 2D materials with hexagonal symmetry during the light-matter interaction, yielding deep insights to attosecond-fast electron dynamics. In addition, we show that a residual ballistic current density is formed, which strongly increases when a band gap is introduced. Both, the sub-cycle electron dynamics and the resulting residual current are relevant for the fundamental understanding and future applications of strongly driven electrons in two-dimensional materials, including graphene or transition metal dichalcogenide monolayers.

Light-Driven Extremely Nonlinear Bulk Photogalvanic Currents

“…We move on to graphene, which is a hexagonal Dirac semi-metal with band touching at the K and K' points. Previous theoretical and experimental work showed that one can generate strongly-driven photocurrents in graphene with monochromatic few-cycle laser pulses [33][34][35][36]. Figure S6 explores current injection from the co-rotating ω-2ω field, showing that similar effects occur in the long-pulse regime (the field is polarized within the graphene sheet).…”

“…Here the photon energies are resonant with 2 nd -and 3 rdorder perturbative transitions that interfere, and the inversion symmetry is effectively broken by the twocolor field (making the effect highly sensitive to the two-color relative phase). The resonant and perturbative nature of these effects precludes access to ultrafast dynamics and possible applications in the Terahertz regime, and is also limited in its conversion efficiency [29].More recently, nonlinear photocurrents were predicted and observed in dielectrics [30][31][32] and graphene [33][34][35][36] driven by intense quasi-monochromatic few-cycle pulses. The mechanism creating these photocurrents relies on the vector potential of the light field to have a nonzero time integral [32], i.e.…”

“…More recently, nonlinear photocurrents were predicted and observed in dielectrics [30][31][32] and graphene [33][34][35][36] driven by intense quasi-monochromatic few-cycle pulses. The mechanism creating these photocurrents relies on the vector potential of the light field to have a nonzero time integral [32], i.e.…”

Quantum Control via Landau-Zener-Stückelberg-Majorana Transitions

“…If the light becomes circularly polarized, the intraoptical-cycle LZSM interference cannot occur and there is no respective change in the current. The intermediate situation takes place for laser pulses with various degrees of ellipticity (Boolakee et al, 2020;Heide et al, 2020Heide et al, , 2019Heide et al, , 2018.…”

Controlling Valley-Polarisation in Graphene via Tailored Light Pulses

“…Also, the idea of ω − 2ω bi-circular fields driven HHG is extended from atomic systems [26,27] to solids [28]. Moreover, remarkable works were reported on electron dynamics in graphene via intense laser pulse [29][30][31][32], including Floquet-engineered valleytronics [33,34].…”